Mass spectrometry (MS) plays an important role in chemical analysis which is currently being enhanced by the increasing demand for rapid trace analysis in the areas of public safety, forensics, food safety and pharmaceutical quality assurance, amongst others. These demands have produced a need to simplify MS instrumentation and methodologies. This in turn has resulted in the development of miniaturized instrumentation (Gao et al., Anal. Chem. 2006, 78, 5994-6002; Cotte-Rodriguez et al., Analyst 2006, 131, 579-589; and Cotte-Rodriguez et al., Anal. Chem. 2008, 80, 1512-1519) and development of ambient ionization methods in which samples are examined without preparation in their native state (Venter et al., TrAC, Trends Anal. Chem. 2008, 27, 284-290; Cooks et al., Biopolymers 2009, 92, 297-297; Ifa et al., Analyst 2010, 135, 669-681; Shiea et al., Rapid Commun. Mass Spectrom. 2005, 19, 3701-3704; Huang et al., Annu. Rev. Anal. Chem. 2010, 3, 43-65; Chen et al., J Am Soc Mass Spectrom 2009, 20, 1947-1963; Law et al., Angew. Chem., Int. Ed. 2009, 48, 8277-8280; Chingin et al., Angew. Chem., Int. Ed. 2010, 49, 2358-2361; and Weston, Analyst 2010, 135, 661-668). Particularly, miniaturized instrumentation is being combined with ambient ionization methods to produce mass spectrometers that can be easily used outside of laboratories.
However, a problem with a system that combines miniaturized instrumentation and ambient ionization is that such a system is limited by the low pumping speed of small mass spectrometers and the large nebulizing gas and solvent volumes that must be handled. This problem has been addressed by the development of a discontinuous atmospheric pressure interface (DAPI; Gao et al., Anal. Chem. 2008, 80, 4026-4032; Gao et al., Int. J. Mass Spectrom. 2009, 283, 30-34; and Gao et al., Int. J. Mass Spectrom. 2009, 283, 30-34). The DAPI interface is opened briefly to admit a bolus of ions, solvent vapor and gas, then closed while the neutrals are pumped away before the trapped ions are mass analyzed.
Even with the implementation of a DAPI, there is still a need to increase the sensitivity and sampling efficiency of systems that combine miniaturized instrumentation and ambient ionization.